Coupling of beta-adrenergic receptor with adenylate cyclase is being studied, both in the native turkey erythrocyte membrane and in reconstituted system of phospholipid vesicles. In the native turkey erythrocyte membranes we found means to change the concentrations of the catalytic unit C, the nucleotide binding protein N, and the receptor R. By studying the kinetics of activation of adenylate cyclase as a function of the concentrations of R, N, C, Gpp(NH)p, and hormone, it became possible to formulate the mechanism of activation as follows: H.R plus NGpp(NH)p.C yield reversibly HR.NGpp(NH)p.C yields HR plus N'Gpp(NH)P.C where the rate limiting step is the bimolecular interaction between HR and N.C. The nucleotide (GTP) binding protein seems to be associated to the catalytic unit C at all times. Also, the binding of the hormone to R and Gpp(NH)p (or GTP) to N are independent events. It was also established that GDP release is not the rate limiting step of the overall process. The rate of reversal of the activated state N'Gpp(NH)p.C back to the basal state N.C (or NGDP.C) is linearly dependent on receptor concentration and, therefore, conforms to the "collision coupling" mechanism, Recent reconstitution experiments show that the Gpp(NH)p activated cyclase and the beta receptor can be incorporated into the same phospholipid vesicles. In these vesicles GTP and 1-epinephrine can induce some reversal.